1. Field of the Invention
The present invention relates to swallowable capsule cameras for imaging of the gastro-intestinal (GI) tract. In particular, the present invention relates to a memory system that is suitable for capsule camera applications.
2. Discussion of the Related Art
Devices for imaging body cavities or passages in vivo are known in the art and include endoscopes and autonomous encapsulated cameras. Endoscopes are flexible or rigid tubes that are passed into the body through an orifice or surgical opening, typically into the esophagus via the mouth or into the colon via the rectum. An image is taken at the distal end using a lens and transmitted to the proximal end, outside the body, either by a lens-relay system or by a coherent fiber-optic bundle. A conceptually similar instrument might record an image electronically at the distal end, for example using a CCD or CMOS array, and transfer the image data as an electrical signal to the proximal end through a cable. Endoscopes allow a physician control over the field of view and are well-accepted diagnostic tools. However, they have a number of limitations, present risks to the patient, are invasive and uncomfortable for the patient. The cost of these procedures restricts their application as routine health-screening tools.
Because of the difficulty traversing a convoluted passage, endoscopes cannot reach the majority of the small intestine and special techniques and precautions, that add cost, are required to reach the entirety of the colon. Endoscopic risks include the possible perforation of the bodily organs traversed and complications arising from anesthesia. Moreover, a trade-off must be made between patient pain during the procedure and the health risks and post-procedural down time associated with anesthesia. Endoscopies are necessarily inpatient services that involve a significant amount of time from clinicians and thus are costly.
An alternative in vivo image sensor that addresses many of these problems is capsule endoscopy. A camera is housed in a swallowable capsule, along with a radio transmitter for transmitting data, primarily comprising images recorded by the digital camera, to a base-station receiver or transceiver and data recorder outside the body. The capsule may also include a radio receiver for receiving instructions or other data from a base-station transmitter. Instead of radio-frequency transmission, lower-frequency electromagnetic signals may be used. Power may be supplied inductively from an external inductor to an internal inductor within the capsule or from a battery within the capsule.
An early example of a camera in a swallowable capsule is described in the U.S. Pat. No. 5,604,531, issued to the Ministry of Defense, State of Israel. A number of patents assigned to Given Imaging describe more details of such a system, using a transmitter to send the camera images to an external receiver. Examples are U.S. Pat. Nos. 6,709,387 and 6,428,469. There are also a number of patents to the Olympus Corporation describing a similar technology. For example, U.S. Pat. No. 4,278,077 shows a capsule with a camera for the stomach, which includes film in the camera. U.S. Pat. No. 6,939,292 shows a capsule with a memory and a transmitter.
An advantage of an autonomous encapsulated camera with an internal battery is that the measurements may be made with the patient ambulatory, out of the hospital, and with only moderate restrictions of activity. The base station includes an antenna array surrounding the bodily region of interest and this array can be temporarily affixed to the skin or incorporated into a wearable vest. A data recorder is attached to a belt and includes a battery power supply and a data storage medium for saving recorded images and other data for subsequent uploading onto a diagnostic computer system.
A typical procedure consists of an in-patient visit in the morning during which clinicians attach the base station apparatus to the patient and the patient swallows the capsule. The system records images beginning just prior to swallowing and records images of the GI tract until its battery completely discharges. Peristalsis propels the capsule through the GI tract. The rate of passage depends on the degree of motility. Usually, the small intestine is traversed in 4 to 8 hours. After a prescribed period, the patient returns the data recorder to the clinician who then uploads the data onto a computer for subsequent viewing and analysis. The capsule is passed in time through the rectum and need not be retrieved.
The capsule camera allows the GI tract from the esophagus down to the end of the small intestine to be imaged in its entirety, although it is not optimized to detect anomalies in the stomach. Color photographic images are captured so that anomalies need only have small visually recognizable characteristics, not topography, to be detected. The procedure is pain-free and requires no anesthesia. Risks associated with the capsule passing through the body are minimal—certainly the risk of perforation is much reduced relative to traditional endoscopy. The cost of the procedure is less than for traditional endoscopy due to the decreased use of clinician time and clinic facilities and the absence of anesthesia.
As the capsule camera becomes a viable technology for inspecting gastrointestinal tract, various methods for storing the image data have emerged. For example, U.S. Pat. No. 4,278,077 discloses a capsule camera that stores image data in chemical films. U.S. Pat. No. 5,604,531 discloses a capsule camera that transmits image data by wireless to an antenna array attached to the body or provided in the inside a vest worn by a patient. U.S. Pat. No. 6,800,060 discloses a capsule camera that stores image data in an expensive atomic resolution storage (ARS) device. The stored image data could then be downloaded to a workstation, which is normally a personal computer for analysis and processing. The results may then be reviewed by a physician using a friendly user interface. However, these methods all require a physical media conversion during the data transfer process. For example, image data on chemical film are required to be converted to a physical digital medium readable by the personal computer. The wireless transmission by electromagnetic signals requires extensive processing by an antenna and radio frequency electronic circuits to produce an image that can be stored on a computer. Further, both the read and write operations in an ARS device rely on charged particle beams.
A capsule camera using a semiconductor memory device, whether volatile or nonvolatile, has the advantage of being capable of a direct interface with both a CMOS or CCD image sensor, where the image is captured, and a personal computer, where the image may be analyzed. The high density and low manufacturing cost achieved in recent years made semiconductor memory the most promising technology for image storage in a capsule camera. According to Moore's law, which is still believed valid, density of integrated circuits double every 24 months. Even though CMOS or CCD sensor resolution doubles every few years, the data density that can be achieved in a semiconductor memory device at least keeps pace with the increase in sensor resolution. Alternatively, if the same resolution is kept, a larger memory allows more images to be stored and therefore can accommodate a higher frame rate.
In a high density memory cell array (e.g., a 1-T DRAM1 or a nonvolatile memory cell), necessary peripheral circuits that are used for addressing the memory cells typically occupy a significant area of the integrated circuit die (“chip”). In a semiconductor memory circuit, addressing signals propagate over metal wires (“buses”) over substantially the entire chip area to connect the large number of decoding circuits required for a large memory array. The addressing signals dissipate an amount of power that may be estimated by the expression P=½ CV2, where V is the supply voltage, and C is the capacitance loading of the signal wires. The capacitance loading has two components: wire capacitance and circuit loading capacitance (e.g., gate capacitance and junction capacitance). As a result, address decoding on a semiconductor memory device is a power-consuming operation. Furthermore, the parallel address lines and the decoding circuits they connect all occupy valuable semiconductor area. 1The data storage portion of an 1-T DRAM cell consists of a single MOS transistor configured as a capacitor.
FIGS. 5A and 5B illustrate an exemplary decoding scheme used in a conventional semiconductor device. For the purpose of illustration only, FIGS. 5A and 5B show 10-bit addresses including a 6-bit row address A12 . . . A7 and a 4-bit column address A3 . . . A0. Even then, the row and column addresses are carried by 20 address lines (i.e., each address bit requires a signal line for itself and its complement), coupled to a large number of decoder logic gates that drive the word lines (FIG. 5A) and the bit lines (FIG. 5B). The memory device shown in FIGS. 5A and 5B is organized as a “×8” device (i.e., each address selects an 8-bit byte). “×1” and “×16” are examples of other popular configurations. In the current generation of semiconductor devices, such as a gigabit device, the complexity is significantly higher. As the requirement for additional address lines and decoding circuits, the required die area and capacitance increase correspondingly. Significant power is therefore dissipated by decoding circuits and conductive paths in a semiconductor device.